Infrared imagers are used in a large number of applications. Infrared imagers generally include an array of pixels. One limitation on the cost of producing infrared imagers is the pixel size. In general, a smaller pixel size allows the imager chip to have smaller dimensions, and thus a lower cost. However, previous manufacturing techniques for infrared imagers based on thermopiles as infrared sensing structures have been too imprecise to produce smaller pixels, for example, pixels with dimensions under 120 μm.
Imaging devices using thermopiles as sensing structures may include a thermopile structure suspended over a cavity in a semiconductor substrate, where a dielectric membrane of the thermopile has openings through to the cavity. Prior manufacturing methods have been challenged in ensuring the proper shape of the cavity under the membrane as well as a much higher precision in aligning this cavity with the position of the thermopile pixel. These features are especially needed when fabricating thermopile pixels which are smaller in size (for example, 120 μm or smaller).
When thermopile structures are integrated as pixels in CMOS integrated infrared imagers, a good insulation and passivation is desirable between the thermopile structures and the integrated circuitry. Previous methods of fabricating the thermopile pixels were based on anisotropic etching from the back of the membrane, which does not address the post etching effects, such as crystal defects, on integrated circuitry. Additionally, the anisotropic etching from the back of the membrane is not generally precise enough to ensure the correct positioning of the cavity under the membrane with high accuracy. For these reasons, a distance of 8-10 μm is desirable between the thermopile structures and CMOS circuitry. This increases significantly the pixel size.
In order to reduce the fabrication cost, it is desirable for infrared imagers to have a pixel size as small as possible, allowing for the imager chip to have smaller dimensions and, thus, a lower cost. Therefore, there is a need in the industry to improve one or more of the abovementioned deficiencies.